ANS:D - Spray towers

Spray towers, also known as spray chambers or spray scrubbers, are air pollution control devices used for gas-liquid contact and mass transfer operations, particularly for the removal of pollutants from gas streams. Here's an explanation of spray towers:

1. Design: A spray tower typically consists of a vertical cylindrical vessel or chamber with spray nozzles or atomizers positioned at the top. The gas stream containing pollutants enters the tower from the bottom and flows upward through the chamber. Liquid, typically water or a chemical solution, is sprayed or atomized into the tower from the top.
2. Gas-Liquid Contact: As the gas stream rises through the tower, it comes into contact with the fine droplets of liquid sprayed from the nozzles. This creates a large interfacial area for mass transfer between the gas and liquid phases. Pollutants present in the gas stream are absorbed or captured by the liquid droplets through processes such as absorption, adsorption, or chemical reaction, depending on the nature of the pollutants and the composition of the liquid.
3. Pollutant Removal: The pollutants captured by the liquid droplets are removed from the gas stream as the droplets settle downward due to gravity. The treated gas stream exits the spray tower from the top, while the liquid containing the captured pollutants is collected at the bottom of the tower and removed for further treatment or disposal.
4. Variations: Spray towers can be designed in various configurations to optimize performance for specific applications. This includes variations in tower diameter, height, nozzle design, liquid flow rate, and the type of packing material used to enhance gas-liquid contact efficiency.
5. Applications: Spray towers are commonly used in industries such as chemical processing, petrochemical refining, pharmaceuticals, wastewater treatment, and air pollution control. They are particularly effective for removing gaseous pollutants such as sulfur dioxide (SO2), hydrogen sulfide (H2S), ammonia (NH3), volatile organic compounds (VOCs), and particulate matter from industrial emissions.

In summary, spray towers are versatile gas-liquid contact devices used for the removal of pollutants from gas streams through absorption, adsorption, or chemical reaction processes. They offer efficient and cost-effective solutions for air pollution control and gas treatment in a wide range of industrial applications.